1. Field of the Invention
The present invention relates to a Group-III nitride semiconductor device comprising a crystal substrate having thereon a Group-III nitride semiconductor (AlXGaYIn1−(X+Y)N: 0≦X<1, 0<Y≦1 and 0<X+Y≦1) crysta layer, and an ohmic electrode having a low contact resistance provided thereon via a low dislocation density boron phosphide crystal layer.
2. Related Art
Known examples of a Group-III nitride semiconductor device include a gallium nitride (GaN)-based light emitting diode (LED) or laser diode (LD) and a Schottky contact-type field-effect transistor (MESFET) (see, for example, Patent Document 1). These devices have a stacked structure comprising a functional layer composed of aluminum gallium indiumnitride (AlaGabIncN: 0≦a,b,c≦1, a+b+c=1) mixed crystal or the like (see, for example, Patent Document 2). For example, a gallium indium nitride mixed crystal (GabIncN: 0<b,c<1, b+c=1) containing GaN having a band gap of about 3.4 eV at room temperature is used as a light-emitting layer in short-wavelength LED or LD denies (see, for example, Patent Document 3). The device is fabricated by providing an ohmic electrode in ohmic contact with a part of the functional layer constituting the stacked structure. For example, titanium (Ti) and aluminum (Al) may be stacked on the surface of an n-type gallium nitride (GaN) electron supply layer to provide ohmic source and drain electrodes, to thereby fabricate a high mobility field-effect transistor (see, for example, Non-Patent Document 1).
Furthermore, the AlaGabIncN (0≦a,b,c≦1, a+b+c=1) mixed crystal constituting the Group-III nitride semiconductor device is conventionally deposited on sapphire (α−Al2O3) as a substrate (see, for example, Patent Document 4). However, a large lattice mismatching is present between the sapphire and the AlaGabIncN (0≦a,b,c≦1, a+b+c=1) mixed crystal or the like. For example, the degree of the lattice mismatching between the sapphire and Wurtzite crystal structure GaN is as large as about 16% (see, Non-Patent Document 2). Therefore, for example, the inside of a gallium nitride layer grown on the sapphire substrate contains a large amount of dislocations in the order of about 1×105/cm2 due to the large lattice mismatching therebetween (see, Non-Patent Document 3).
Patent Document 1
U.S. Pat. No. 6,069,021, specification
Patent Document 2
JP-A-10-56202 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)
Patent Document 3
JP-B-55-3834 (the term “JP-B” as used herein means an “examined Japanese patent publication”)
Patent Document 4
JP-A-10-107315
Non-Patent Document 1
Isamu Akasaki (compiler), Advanced Electronics Series, I-21, Group-III Nitride Semiconductor, 1st edition, pp. 288–289, Baifukan (Dec. 8, 1999)
Non-Patent Document 2
Isamu Akasaki, Hiroshi AMANO, Yasuo KOIDE, Kazumasa HIRAMATSU and Nobuhiko SAWAKI, “Effects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Gal-XAlXN (0<X≦0.4) Films Grown on Sapphire Substrate by MOVPE”, Journal of Crystal Growth (the Netherlands), Vol. 98, pp. 209–219 (1989).
Non-Patent Document 3
Isamu Akasaki (compiler), Advanced Electronics Series, I-21, Group-III Nitride Semiconductor, 1st edition, pp. 211–213, Baifukan (Dec. 8, 1999)
However, the band gap, for example, of hexagonal Wurtzite crystal structure GaN is as high as about 3.4 eV at room temperature and the Group-III nitride semiconductor (AlaGabIncN: 0≦a,b,c≦1, a+b+c=1) mixed crystal layer on which an ohmic contact electrode is generally provided has a high band gap.
Because of this, an ohmic electrode having sufficiently low contact resistance can hardly be obtained. Furthermore, the AlaGabIncN crystal layer grown on a sapphire substrate has a problem in that device operating current can leak through dislocation present with a high density in the crystal. As a result, an ohmic electrode having excellent breakdown voltage cannot be formed.